A spectroscopic map of the Galactic centre

Observations and resolved stars

¹ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Wien, Austria
² Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
³ Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁵ ESA–ESAC–European Space Agency, Camino Bajo del Castillo s/n, 28692 Villafranca del Castillo, Madrid, Spain
⁶ European Space Research and Technology Centre, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
⁷ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova, Vicolo dell’Osservatorio 3, Padova 35122, Italy
⁸ Technion – Israel Institute of Technology, Haifa 3200002, Israel
⁹ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain

^★ Corresponding author; anja.krause@univie.ac.at

Received: 12 December 2024
Accepted: 10 March 2025

Abstract

Context. The Galactic centre (GC) region contains a dense accumulation of stars that can be separated into two components: a mildly flattened and extremely dense nuclear star cluster (NSC) and a surrounding more extended and more flattened nuclear stellar disc (NSD). Previous studies have collected a few thousand spectra of the inner NSC and the outer NSD and have measured line-of-sight velocities and metallicities. Until now, such measurements exist only for a few hundred stars in the region where the stellar surface density transitions from being dominated by the NSC to being dominated by the NSD.

Aims. We seek to study the stellar population from the centre of the NSC out to well beyond its effective radius, where the NSD dominates. In this way, we can investigate whether and how the mean properties and kinematics of the stars change systematically.

Methods. We conducted spectroscopic observations with Flamingos-2 in the K-band via a continuous slit scan. The data extend from the central NSC to the inner NSD, out to ±32 pc from Sgr A^★ along Galactic longitude l. Based on their CO equivalent widths, we classified the stars in these areas as hot or cool stars. The former are massive young stars, while almost all of the latter are older than one to a few gigayears. Applying full-spectral fitting, we measured the overall metallicity [M/H] and line-of-sight velocity V_LOS for more than 2500 cool stars, increasing existing samples outside of the very centre by a factor of three in terms of the number of stars and by more than an order of magnitude in terms of covered area. We present the first continuous spatial maps and profiles of the mean value of various stellar and kinematic parameters.

Results. We identify hot young stars across the field of view. Some stars appear to be isolated from other hot stars, while others accumulate within 2.7 pc of the Quintuplet cluster, or the central parsec cluster. The position-velocity curve of the cool stars shows no dependence on [M/H], but it depends on the colour of the stars. The colour may be a tracer of the line-of-sight distance and thus distinguish stars located in the NSC from those in the NSD. A subset of the cool stars has high velocities (i.e. greater than 150 km s⁻¹), and they may be associated with the bar or tidal tails of star clusters.